Mirror modifier prevents me from weight painting
i just tried it out - and it worked exactly how i would have expected it: yes, you can just weight paint on the one side, like everything you can do only on the one side of the mirrored object. That's why it's called mirrored....you can also just change the vertices on one side...and so on.
You can apply the mirror modifier (just in object mode):
You don't need to apply the modifier, only if one side should not be mirrored anymore or you wanna change something just on one side.